Process for producing artificial staple fibers



United States Patent 0 3,420,814 PROCESS FOR PRODUCING ARTIFICIAL STAPLE FIBERS Otto Reichert, Kurt Heuer, and Heinz Grotjahn, Oberbruch-Grebben, and Hans George Wendlandt, Oberhruch, Germany, assignors to American Enka Corporation, Erika, N.C., a corporation of Delaware No Drawing. Filed July 21, 1965, Ser. No. 473,847 Claims priority, applicatior Germany, July 31, 1964, 2 US. Cl. 264-197 3 Claims Int. Cl. Dtllf 3/12, 3/28 ABSTRACT OF THE DISCLOSURE A high strength, easy separable and processible rayon staple fiber is produced by spinning continuous filaments into a coagulating bath containing a small amount of formaldehyde and no heavy metal salts, by subsequently stretching the filaments in a second bath at a controlled temperature, by cutting the filaments into staple fiber while still in an incompletely regenerated state, i.e., at a gamma number of 35 to 47, and by depositing the cut fibers into an acid floating bath maintained at a tem* perature above 90 C.

This invention relates to a process for the manufacture of regenerated cellulose structures from viscose spinning solutions. More particularly, it relates to a process for producing regenerated cellulosic staple fibers which renders them more suitable for subsequent textile processing and blending with other type fibers.

The method of converting freshly spun continuous filament yarns directly into staple fiber products by submitting a bundle of the continuous filaments to the action of a cutting surface is well known. In the production of regenerated viscose fibers, viscose is normally extruded in continuous filament form through a spinnerette orifice into an acid coagulating bath. The bath is composed of an equeous solution containing amounts of sulfuric acid, sodium sulfate, and heavy metal salts such as zinc sulfate. Various viscose modifiers may be added either directly to the viscose or to the coagulating bath to produce desired effects in the final product.

In the manufacture of viscose rayon staple fibers, the continuous filaments, after being stretched and regenerated in the acid bath, are cut into the desired staple length by subjecting the continuous filaments to cutting or breaking devices. The bundles of staple fibers are then washed and tumbled to open the bundles into discret, individual fibers. Rayon fibers, which have an appearance similar to those natural fibers of cotton and wool, are then transformed by standard carding operations into yarns which are subsequently woven or knitted into textile articles. Quite often, rayon fibers are blended with dissimilar fibers such as wool and cotton to produce novel textile effects.

To take advantage of the favorable properties of the individual fibers in the manufacture of high quality'textile articles, it is necessary to produce fibers which are in a form that can be readily cardable into yarn. Early attempts were made to produce rayon staple fibers having a high capacity for adhesion when combined in a carding operation. In US. Patent 2,254,777, staple fiber is produced by spinning a highly ripened (low gamma number), continuous filament viscose into an alcohol spin bath where the viscose is coagulated. The filaments are cut into staple length prior to complete decomposition and are subsequently decomposed by means of a hot salt solution. By cutting the filaments prior to complete de- 3,420,814 Patented Jan. 7, 1969 "ice composition of the cellulose xanthate to cellulose, the individual fibers shrink, crimp, and develop a surface structure which has a high capacity of adhesion when the fibers are subsequently processed into textile materials. Fibers produced in such a manner are not feasible for use in present textile operations because they have very low strength and the alcohol regeneration system is too costly for economical production.

Under present day requirements of greater strength in regenerated cellulose filaments and fibers, it is customary to subject freshly spun filaments to a two-bath spinning process which utilizes, in addition to a normal coagulating spinbath, a second bath maintained at a higher temperature and lower sulfuric acid content. In this bath a high degree of stretch is imparted to the filaments to orient the molecules of the cellulose and produce higher strength in the product. After stretching, the yarn is washed, desulfurized and dried in conventional manner. A stronger viscose yarn can be produced in the two-bath system when a small amount of formaldehyde is added to the acid spinbath to increase the stretchability of the yarn. The use of formaldehyde in a two-bath spinning system to improve strength of viscose yarn is disclosed in US. Patent 2,452,130, commonly assigned with thepresent application. More recently, it has been found (US. Patent 2,937,070) that a high strength rayon yarn can be produced by using formaldehyde in the coagulating bath, maintaining the coagulating bath free of heavy metal salts, e.g., Zinc sulfate, and stretching the coagulated filaments to the maximum extent in the second regenerating bath. By controlling the composition and conditions of the coagulating bath, the rate of change of the cellulose xanthate during regeneration can be controlled. The use of formaldehyde and removed of heavy metal salts from the spinbath prolongs the regeneration step and enables the coagulated filaments to be stretched to a greater extent before complete regeneration occurs. Stretching the filaments produces greater molecular orientation along the filament axis and consequent increased strength in the yarn product.

In production of high strength staple fibers by the above method, difficulties are encountered because the filaments adhere to each other after cutting and are not easily separable into individual staple fibers. In rayon staple production, it is highly desirable that the cut bundles be opened as much as possible, not only to simulate natural fibers such as wool and cotton, but to enable the rayon fibers to be more easily blended with other fibers for production of textile material. Because of the difiiculties of fiber adhesion and incomplete bundle separation, high strength rayon fibers are difiicult to process into textile materials.

It is an object of the present invention to provide an improved process for the production of high strength rayon staple fibers which have high elongation, do not adhere during regeneration, and are quickly separable into individual fiber components.

It is a further object of this invention to provide a high strength, high swelling rayon staple fiber easily processible into subsequent textile blends and materials.

The manner in which these and other objects of the invention are attained will become apparent from the following description of the invention, which is intended to be illustrative rather than limitative.

According to the present invention, it has been discovered that a high strength rayon staple fiber can be cut from continuous filament to produce easily separable and processible staple fiber components if the filaments are spun into a coagulating bath containing a small amount of formaldehyde and no heavy metal salts, are subsequently stretched in a second bath at a controlled temperature and are cut into staple fibers while still in an incompletely regenerated state, i.e., at a gamma number of 3547. When the fibers leaving the cutter have such a gamma number, the non-decomposed xanthate in the fibers is evolved as a gas when the fibers are subsequently washed in an acid floating bath. This gaseous release creates a somewhat explosive action to loosen the bunches of fibers so that the individual fibers are separated and may freely shrink and crimp; In contrast to the processes of the prior art in which a tow of high tenacity yarn is spun, stretched and completely regenerated in a hot acid second bath, before being cut into staple fibers, the present process utilizes the gas evolved during final decomposition, after cutting, to explode the bundles of cut fibers into a highly separated and easily processible staple product. The present process is carried out on a composition of viscose having a high gamma number i.e., about 80, which is spun :at low temperature into a precipitation bath. The bath contains, in addition to the normal amounts of sulfuric acid and sodium sulfate, a low percentage of formaldehyde. For optimum results, the bath should not contain heavy metal salts. The formaldehyde reacts in a normal way to slow down the speed of decomposition of the cellulose xanthate. This enables the threads to be more highly stretched and strengthened in the second bath.

The spun threads, which may be partially stretched in air, are finally stretched to about 280300% in a second, weakly acid bath having a sulfuric acid content of about 20 g./kg. The second bath must be maintained at a temperature of between 78 and 85 C. After stretching the filaments are cut into staple length while they are still in the partially regenerated state. It is critical to the present invention that the staple fibers, upon leaving the cutter, have a gamma number between 35 and 47, and preferably higher than 40. The staple fibers leaving the cutter drop into an acid floating bath maintained at a temperature above 90 C. Because of the non-decomposed xanthate remaining in the fibers, :an amount of gas is evolved as the fiber bundles float on the bath. This gaseous liberation explosively separates the bunches of fibers and enables the individual fibers to freely shrink and crimp in the floating bath. It has been found through experimentation that, if the upper gamma number limit of the cut fibers is exceeded, the fibers tend to stick to one another. Thus, by keeping the gamma number below 47 at the cutter, the fibers do not stick in the washing bath.

To maintain a gamma number of the fibers between 35 and 47 at the cutter, the viscose solution should have a very high gamma number at initial spinning, i.e., about 80. It is well known that the presence of formaldehyde in the spinning bath slows the decomposition rate of the cellulose xanthate so that it becomes, to a great extent, independent of the sulfuric acid and sodium sulfate content of the spinning bath. As a result of the use of formaldehyde, the spinning bath composition can be varied within broad ranges, making it possible to adapt the content of the bath to a given alkali content of the viscose or to particular conditions of the spinning apparatus. If spinning is carried out at low temperatures, it is possible to produce a thread having a gamma number which is high enough that the subsequent partial regeneration in the second, or stretch bath does not lower the gamma number below the minimum 35 required at the cutter. This is in contrast to the prior art spinning conditions in which the thread leaving the second bath is generally at a gamma number between 20 and 25.

The importance of controlling the gamma number of the fibers at the cutter and the temperature of the secondary bath, which must be maintained between 78 and 85 C., becomes particularly critical when the staple fibers, after being regenerated and washed, are treated with caustic soda. If the temperature range in the second bath is exceeded, the resulting gamma numbers of the staple fibers are either too high or too low and during caustic soda treatment the fibers will strongly adhere to one another. Also such fibers treated with caustic soda, when subjected to the usual finishing treatments, remain in bunches of non-crimped fibers which can not be formed into a web on a carding machine.

The caustic soda treatment of staple fibers is Well known and is used to produce higher elongation. Such treatment is most desirable when the staple fibers are to be blended, during further processing, with fibers of different origin. When the caustic is used, it replaces the usual desultfurizing treatment. The treatment consists of subjecting the fibers to a aqueous sodium hydroxide bath maintained at a temperature of about 25 C. and a concentration of -55 g./kg. These limits can be varied depending on the degree of elongation required. The treatment lasts about 30 to 60 seconds.

When employing treatment with caustic soda, the strength and swelling value of the fibers of the present invention are not appreciably affected.

Spun yarns and fabrics produced from the rayon staple fibers of the present invention compare quite favorably with and are, in most instances, superior to corresponding spun yarns and fabrics of combed American cotton. The following chart shows the comparative data between the staple fiber of the present invention and that of cotton.

Shirting fabric of the usual construction:

Shrinkage after Dry strength (warp) washes, percent Warp Weft Rayon yarn 19.2 break. length (km.) 2. 3 -2. 8 Cotton yarn 16.5 break. length (k1n.) 8. 8 2. 0

By way of further illucidation of the invention, the following examples are given. The examples illustrate the effect produced on the staple fiber by varying the conditions within and without the limits of the invention.

Example I Beech-wood cellulose was processed in the usual manner to produce a viscose containing 6.2% cellulose and 8.0% NaO-H. The amount of CS added was calculated on the cellulose. The ripening time was chosen to produce a viscose gamma number of 80 at spinning. The viscosity during spinning was 280 seconds (ball fall method) at 20 C. The viscose was spun through a spinnerette having 16,000 orifices 60 microns in diameter into a spinning bath containing g./kg. sulfuric acid, 108 g./ kg. sodium sulfate, and 8.3 g./kg. formaldehyde. The temperature of the bath was maintained at 31 C. The tow leaving the primary coagulation bath was stretched in the air and further stretched in a second bath having an acid content of about 20 g./kg. The tow was stretched a total of 283%. The temperature of the second bath was maintained at 79 C. Upon leaving this stretch bath, the tow was cut into staple fibers 40 mm. in length and floated in a hot acid regenerating bath. The gamma number of the staple fibers leaving the cutter was 42. The temperature of the regenerating bath was maintained at C. and the residence time of the fibers in the bath was 60 seconds. The fibers were Washed on a screen belt, desulfurized in a 2.0 g./ kg. caustic soda bath maintained at 65 C., further washed, bleached and lubricated. The staple fibers produced had a 1.25 filament denier, a conditioned strength of 5.7 g./ den. at an elongation of 7%, a wet strength of 3.9 g./ den. at an elongation 7%, and a swelling value of 58%.

Example II Viscose prepared as in Example I was spun through a spinnerette having 3,600 orifices of 60 micron diameter into a spinning bath containing 70 g./ kg. sulfuric acid, 80 g./kg. sodium sulfate, and 7.8 g./kg. formaldehyde. Subsequent conditions were the same as in Example I with the exception that the thread was stretched to a total of 364%. The staple fiber produced had a filament denier of 1.38, a conditioned strength of 6.9 g./ den. at an elongation of 6.9%, and a wet strength of 5.1 g./ den. at an elongation of 7.6%.

Example III Staple fiber was produced as indicated in Example I. Instead of the desulfurizing treatment, the fiber bunches were subjected to a bath containing 45.0 g./ kg. NaOH at 25 C. for 30 seconds. The fibers were washed, bleached and lubricated in the usual manner. The fibers had a 1.27 filament denier, a conditioned strength of 5.1 g./ den. at 9.3% elongation, a wet strength of 3.5 g./den. at an elongation of 8.9%, and a swelling value of 65%.

Example IV A staple fiber was prepared as indicated in Example I. Instead of the desulfurizing treatment, the fiber bunches were subjected to a bath containing 56.8 g./ kg. NaOH at 25 C. for 30 seconds. The fibers were then washed, bleached and lubricated in the usual manner. The fibers produced had a 1.35 filament denier, a conditioned strength of 4.7 g./ den. at 12.9% elongation, a wet strength of 3.4 g./den. at an elongation of 12.3%, and a swelling value of 75%.

Example V Viscose was prepared and spun and the tow obtained stretched in the air as indicated in Example I. The tow was then stretched in a tempered acid bath at a temperature of 74 C. The acid content of the bath was g./kg. The tow was stretched to a total of 283%, out into staple fibers, and the fibers were floated in a bath having an acid content of 20 g./kg. and a temperature of 95 C. The fibers leaving the cutter had a gamma number of 50. The fibers were washed, treated with caustic soda as described in Example III, bleached, and lubricated in the usual manner. The fibers obtained, during flotation and regeneration, coalesced and became so felty that further processing and evaluation of the fiber properties was impossible. It is seen from the foregoing example that if the temperature of the stretch bath is maintained outside the limits of 78 to 85 C., the gamma number of the fibers at the cutter is lowered, producing an unprocessible fiber.

Example VI Viscose manufactured and spun as in Example I was stretched in air and in a 20' g./ kg. acid bath at 92 C. The tow was cut into staple fibers and float regenerated in a bath at 95 C. having an acid content of 20 g./kg. The gamma number of the fibers at the cutter was 18. After washing, the fibers were treated with caustic soda as in Example III, bleached and lubricated in the usual manner. The resulting staple fiber had a filament denier of 1.28, a conditioned strength of 5.0 g./den. at an elongation of 9.1%, a wet strength of 3.5 g./den. at an elongation of 6 18.9%, and a swelling value of The fibers obtained could not be processed into a yarn because the absence of crimp in the individual fibers prevented the formation of a web on a carding machine.

It can be seen from the foregoing examples and descrip tions of the invention that by use of the present invention it has become possible to produce satisfactorily processible staple fibers of regenerated cellulose when spinning viscose at a high gamma number into zinc-free formaldehyde containing acid baths. The excellent textile and swelling properties of these fibers are utilized to manufacture high-quality yarns spun from the staple fibers.

While specific examples of the preferred methods of the present invention have been described, it will be apparent that many changes and modifications may be made therein without departing from the spirit of the invention. It is, therefore, understood that the invention is to be limited only by the extent of the following claims.

What is claimed is:

1. A method of producing a high strength viscose rayon staple fiber having improved separability after cutting and ease of processing into textile yarns and fabrics which comprises the steps of extruding 'a plurality of viscose filaments having a gamma number of about into a first acid coagulating bath free of heavy metal salts and containing a small amount of formaldehyde, pasing the thus coagulated filaments into a second bath having a lower acid content that said first coagulating bath where they are stretched and partially regenerated, cutting the stretched filaments into staple length fibers, and subjecting the cut fibers to a weak acid solution to bring about complete regeneration thereof and to loosen bundles of fibers so that the individual fibers are separated in order to freely shrink and crimp by maintaining the said second bath at a temperature between about 78 C. and C., maintaining the gamma number of the cut fibers between 35 and 47 at the beginning of their regeneration in the weak acid solution, and maintaining the temperature of the weak acid solution above C.

2. A method as defined in claim 1 in which the gamma number of the cut fibers is maintained above 40 at the beginning of their regeneration in the weak acid solution.

3. A method as defined in claim 1 in which the staple fibers, after regeneration, are washed and treated with a dilute caustic soda solution.

References Cited UNITED STATES PATENTS 3,077,374 2/ 1963 Bates 264-198 X 3,109,698 11/19 63 Klein et al. 264-197 3,324,216 6/ 1967 Inoshita et al. 264-197 3,352,957 11/1967 Drisch et al. 264-191 JULIUS FROME, Primary Examiner.

J. H. WOO, Assistant Examiner.

US. Cl. X.R. 

